1. Field of the Invention
The present invention relates generally to the field of medical devices, and, more specifically, to sensor structures for medical implants.
2. Related Art
The combination of biosensors and microelectronics has resulted in the availability of portable diagnostic medical equipment that has improved the quality of life for countless people. Many people suffering from disease or disability who, in the past, were forced to make routine visits to a hospital or doctor's office for diagnostic testing currently perform diagnostic testing on themselves in the comfort of their own homes using equipment with accuracy to rival laboratory equipment.
Nonetheless, challenges in the biosensing field have remained. For example, although many diabetics currently utilize diagnostic medical equipment in the comfort of their own homes, the vast majority of such devices still require diabetics to draw their own blood and inject their own insulin. Drawing blood typically requires pricking a finger. For someone who is diagnosed with diabetes at an early age, the number of self-induced finger pricks over the course of a lifetime could easily reach into the tens of thousands. In addition, the number of insulin injections may also reach into tens of thousands.
Some medical conditions have been amenable to automated, implantable sensing. For example, thousands of people with heart conditions have had pacemakers or defibrillators implanted into their bodies that utilize sensors for monitoring the oxygen content of their blood. Ideally, these sensors should be able to determine whether, for example, a person's heart is running very efficiently at a high heart rate or whether a person's heart has entered fibrillation. In order to effectively make this determination, an accurate sensor must be employed. Unfortunately, oxygen sensors implanted into the body have, thus far, typically required frequent and periodic recalibration and replacement.
An important aspect of implantable sensors, regardless of the application, is the integrity of the sensor structure itself. When the integrity of the sensor structure fails, either partially or completely, undesired fluids and cells may diffuse into the sensor structure, causing unstable sensing performance or sensor failure. For example, once the integrity of an implanted sensor has become compromised, blood may pool and clot around electrodes, fibrous growths may develop over the top of electrodes, and softer elements (e.g., enzymes, protein matrices, membranes and the like) of the sensor may deteriorate or be digested.
Once a sensor structure has been compromised in this manner, the sensor must be replaced, requiring an explantation procedure for the old sensor and an implantation procedure for a new sensor. These surgical procedures are semi-invasive, expensive and inconvenient. Further, unless the sensor fails completely, it may be difficult to detect the exact status of the sensor without explanting it first.
In clinical experiments, complications have arisen during implantation of a sensor due to the flexibility of the sensor structure. A surgeon may find that the sensor lead buckles under the axial pressure exerted by the surgeon to force the sensor through the introducer, particularly if a kink is formed in the introducer. As an example, the introducer 140 shown in FIG. 11 has a 45 degree kink 142 formed therein. As an insufficiently stiff and/or lubricious sensor is advanced through the introducer 140, the kink 142 may obstruct the sensor and cause it to buckle in the lateral direction (shown by arrow 144) rather than moving forward in the axial direction of the introducer 140 (shown by arrow 146). As a result of buckling, the sensor may be damaged.
A further complication exists in that the surgeon, while placing the sensor into the introducer, may grip the sensor lead 150 away from the tip where the sensor 152 is located and towards the proximal end of the sensor lead, as shown in FIG. 12A where the gripping point is represented by a pair of crosshatched boxes 154, 156. As a result of the insufficient stiffness of the sensor lead 150, it may hang in a flaccid state when gripped in this manner, making it difficult to place into the introducer. Thus, the surgeon may instead grip the sensor lead 150 closer to the tip, as shown in FIG. 12B, in order to more easily place the sensor lead 150 into the introducer 100 (FIG. 11). However, while gripping the sensor lead 150 in this manner, the surgeon may inadvertently damage the sensor, for example, by exerting excessive pressure on the sensor or because the sensor is made from fragile materials.
Yet another complication resulting from an insufficiently stiff sensor, as shown in FIG. 13, is that when inserted in a vessel 162, the sensor 152, under pressure from fluid flowing through the vessel 162, may rest against a side-wall 166 of the vessel 162 rather than near the center of the vessel 162. This may result in a reduction in the accuracy of the sensor readings or may cause vessel irritation.
Accordingly, there is an industry need for a sensing apparatus that may be implanted into the body and that may remain in the body reliably for extended periods of time. There is a further need in the industry for a sensor structure having enhanced integrity to ensure long-term protection of the sensitive elements of the sensor structure. There is a further need in the industry for a sensor structure that is sufficiently stiff to facilitate steerability of the sensor structure into the body. There is a further need in the industry for a sensor structure that has a surface that is lubricious enough to facilitate implantation of the sensor structure into the body.